RF power amplifiers are used in a wide variety of communications and other electronic applications. These amplifiers are made up of one or more cascaded amplifier stages, each of which increases the level of the signal applied to the input of that stage by an amount known as the stage gain. Ideally, the input to output transfer of each stage is linear; a perfect replica of the input signal increased in amplitude appears at the amplifier output. In reality, however, all power amplifiers have a degree of non-linearity in their transfer characteristic. This non-linearity adversely effects various amplifier operating characteristics such as gain performance, intermodulation performance and efficiency.
Non-linear amplifier transfer characteristics gives rise to a phenomenon, hereinafter referred to as gain expansion. Gain expansion is caused by the change in the amplifier's base-emitter voltage due to rectification of input signal power in the base-emitter junction. In effect, the input signal power to an RF amplifier changes the amplifier's quiescent operating point. As a result, an RF amplifier's gain will increase as a function of the input signal power, thereby giving rise to the gain expansion phenomenon. Gain expansion is typically an undesirable characteristic exhibited by RF power amplifiers. This is especially true when the amplifier must operate across a wide dynamic range of input signals, like the multi-tone linear power amplifiers disclosed in patent application Ser. No. 07/855,172, filed Mar. 20, 1992, entitled HIGH DYNAMIC RANGE MODULATION INDEPENDENT FEED FORWARD AMPLIFIER NETWORK and assigned to the assignee of the present application. In such multi-tone applications, constant amplifier gain over a wide dynamic range of input signals is required.
This same non-linearity causes distortion of the amplifier's output signal so that it is no longer a perfect replica of the input signal. This distortion produces spurious signal components known as intermodulation products. Intermodulation products are typically undesirable because they cause interference, cross talk, and other deleterious effects on the performance of a system employing the amplifier. Of note, the quantity of intermodulation products generated by the amplifier is directly proportional to the magnitude of the signal applied to the amplifier's input.
Yet another RF power amplifier operating characteristic hampered by non-linear transfers is the amplifier's efficiency. By definition, an amplifier's efficiency is determined by POUT/PIN. The more efficient an amplifier is , the less input power required to achieve a desirable output level. Since gain expansion tends to distort the amplifier's output power level, it has the undesirable effect of decreasing an amplifier's efficiency at low output powers.
Accordingly, the prior art reflects various methods and devices designed to improve one or more of the amplifier's operating characteristics, typically at the expense of others. As will be appreciated, optimizing for any one parameter adversely effects the others, since they are all closely interrelated. Thus, while biasing the amplifier's quiescent operating point low tends to improve the amplifier's efficiency, intermodulation performance and saturation point, it nonetheless compromises the maximum gain available and the amplifier's gain flatness (constant gain over wide dynamic range). Conversely, while biasing the amplifier's quiescent operating point higher tends to improve the maximum gain available and the amplifier's gain flatness, it nonetheless compromises the amplifier's efficiency, intermodulation performance and saturation point.
It would be extremely advantageous therefore to provide a compensation circuit capable of equalizing an RF power amplifier's operating characteristics over a wide dynamic range of inputs, while avoiding the shortcomings of the prior art.